Imagine it's 1949, and you're a cop, and you get the assignment to form a vigilante squad of five or six men to bring down the empire of the biggest gangster in Los Angeles, Mickey Cohen. What would you do?

One obvious strategy would be sabotage. Find his assets, go there after hours, and start blowing things up. Keeping your identities secret would be essential, so you wouldn't do anything out in the open.

And with only six guys, you would never walk into one of his establishments with revolvers and shoot it out against 20 gangsters with machine guns. That would be stupid, wouldn't it?

And yet that is precisely what the cops in “Gangster Squad” keep doing, so they must be stupid, and the filmmakers must expect us not to notice.

Here's the problem: A movie like this — set in the film noir days of postwar Los Angeles — gains a lot from its historical authenticity (or even the illusion of it). But by the second or third time the gangster squad provokes some insane public gunfight with Cohen's army, you catch on that you're not seeing something real or almost real but an attempt to ramp up an old story for maximum action.

With that realization goes the illusion of this movie's integrity. What's left is nothing terrible, just typical.

We first meet the villain as he snarls into the camera, and the narrator tells us this is Mickey Cohen — as everyone in the audience thinks, “No, actually, that's Sean Penn.”

This is the challenge any well-known actor assumes when he takes on a character role, but Penn's provides so much psychological detail that Cohen becomes fascinating — evil to the point of demonic but also tense, bitter, resentful, bottled-up and strange.

Penn succeeds in this despite a script that has him acting like a one-note James Bond villain. On three occasions, an underling screws up something and apologizes, and Cohen accepts the apology — and then immediately has the person killed.

In the funniest example (but was it intended to be funny?), he says to his henchmen, “You know the drill.” Whereupon, a huge drill is produced, and blood is sprayed everywhere. It's something out of “Monty Python.” We're to assume that drills are kept in every room, just in case he chooses to use that expression.

By now, you might have inferred that “Gangster Squad” is an exceedingly violent movie, from an early scene in which a man is chained to two cars and torn in half to the climactic machine-gun battle at a hotel. The film was scheduled to open in September, but following the mass killings in Aurora, Colo., a scene depicting people getting machine gunned inside a movie theater had to be cut and reshot. Now the machine guns are in Chinatown.

Director Ruben Fleischer leans hard on the violence card, but with little return in the way of excitement, and at the expense of character and story.

Josh Brolin plays the leader of the gangster squad as a kind of dedicated dunce, which is appropriate considering their clumsy antics. Ryan Gosling has more nuance as his right-hand man, but Emma Stone is completely out of her element as a slinky film noir heroine, a walking anachronism.

Speaking of anachronisms, at one point Stone stands up from her table at a nightclub and says, “I think I'll have a cigarette,” and in another scene, a gangster is shown smoking on a balcony. Historical footnote: Until around 1980, people actually went inside to smoke and outside to breathe.